BRAM frame buffer support

[brianbennett]

Instead of computing pixel color as they are outputted, ppu should compute pixel color and dump into a "back buffer" in block ram. This will enable a couple nice things:

Perfect PPU timings. Currently the PPU timings are kind of close, but are based on the VGA signal requirements. By decoupling the color generation and the display, the color generation should be able to work at a different clock than the display.

Frame capture for automated testing.